A - Used on climate outlooks to indicate areas that are likely to be above normal.
Anomaly - The deviation of a measurable unit, (e.g., temperature or precipitation) in a given
region over a specified period from the long-term average, often the thirty year mean, for the
AO - See Arctic Oscillation
Apparent temperature - The temperature people perceive under hot and humid conditions.
(See Excessive Heat Outlook).
Arctic Oscillation (AO) - The Arctic Oscillation is a pattern in which atmospheric
pressure at polar and middle latitudes fluctuates between negative and positive phases. The
negative phase brings higher-than-normal pressure over the polar region and lower-than-normal
pressure at about 45 degrees north latitude. The negative phase allows cold air to plunge into the
Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The
positive phase brings the opposite conditions, steering ocean storms farther north and bringing
wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as
California, Spain and the Middle East. The North Atlantic Oscillation is often considered to be a
regional manifestation of the AO.
Atmospheric Circulation Model - A mathematical model for quantitatively describing,
simulating, and analyzing the structure of the circulation in the atmosphere and the underlying
causes. Sometimes referred to as Atmospheric General Circulation Models or AGCMs (See
B - Is used on climate outlooks to indicate areas that will likely be below normal.
Chlorofluorocarbons (CFCs) - Manufactured substances used as coolants and computer-chip
cleaners. When these products break down they destroy stratospheric ozone, creating the
Antarctic Ozone Hole in the Southern Hemisphere spring (Northern Hemisphere fall). While no
longer in use, their long lifetime will lead to a very slow removal from the atmosphere.
Circulation - The flow, or movement, of a fluid (e.g., water or air) in or through a given area or
CL - An abbreviation used on climate outlook maps to indicate areas where equal chances of
experiencing below-normal, normal, or above-normal conditions are possible.
Climate - The average of weather over at least a 30-year period. Note that the climate taken over
different periods of time (30 years, 1000 years) may be different. The old saying is climate is
what we expect and weather is what we get.
Climate Change - A non-random change in climate that is measured over several decades or
longer. The change may be due to natural or human-induced causes.
Climate Diagnostics Bulletin (CDB) - The monthly CPC Bulletin reports on the previous
months' status of the ocean-atmosphere climate system and provides various seasonal ENSO-related outlooks. It is issued by the fifteenth of the month.
Climate Diagnostics Center (CDC) - The mission of NOAA's Climate Diagnostics Center is to
identify the nature and causes for climate variations on time scales ranging from a month to
centuries. (See http://www.cdc.noaa.gov)
Climate Model - Mathematical model for quantitatively describing, simulating, and analyzing the
interactions between the atmosphere and underlying surface (e.g., ocean, land, and ice).
Climate Outlook - A climate outlook gives probabilities that conditions, averaged over a
specified period, will be below-normal, normal, or above-normal.
Climate Prediction Center (CPC) - This Center is one of several centers under the National
Centers for Environmental Prediction (NCEP) part of the National Weather Service (NWS) in
the National Oceanic and Atmospheric Administration (NOAA). The Center serves the public
by assessing and forecasting the impacts of short-term climate variability, emphasizing enhanced
risks of weather-related extreme events, for use in mitigating losses and maximizing economic
Climate System - The system consisting of the atmosphere (gases), hydrosphere (water),
lithosphere (solid rocky part of the Earth), and biosphere (living) that determine the Earth's
Climate Test Bed - The Climate Test Bed is an NCEP project, led by CPC in close
collaboration with EMC and open to the climate community inside and outside NOAA. Its mission is to accelerate the transition of research
and development into improved NOAA operational climate forecasts, products, and applications. It will support and promote: exchange of
climate forecast system software among NCEP, GFDL, NASA, NCAR, COLA, and other potential contributors; multi-model ensembles; community-
based, reviewed proposals for specific scientific priorities; potential expansion to experimental use of ECMWF, Canadian, Met
Office, MeteoFrance products.
Climatological Outlook - An outlook based upon climatological statistics for a region,
abbreviated as CL on seasonal outlook maps. CL indicates that the climate outlook has an equal
chance of being above normal, normal, or below normal.
Climatology - (1) The description and scientific study of climate. (2) A quantitative description
of climate showing the characteristic values of climate variables over a region.
Composite - An average that is calculated according to specific criteria. For example, one might
want a composite for the rainfall at a given location for all years where the temperature was much above average.
Condensation - The physical process by which water vapor in the atmosphere changes to liquid
in the form of dew, fog or cloud; the opposite of evaporation.
CONUS - Continental United States
Convection - Transfer of heat by fluid motion between two areas with different temperatures. In
meteorology, the rising and descending air motion caused by heat. Atmospheric convection is
almost always turbulent and is the dominant vertical transport process over tropical oceans and
during sunny days over continents. The terms "convection" and "thunderstorms" are often use
interchangeably, although thunderstorms are only one form of convection. In the ocean,
convection is prominent in regions of high heat loss to the atmosphere and is the main
mechanism for deep water formation.
Cooling Degree Days - A form of degree-day used to estimate energy requirements for air
conditioning or refrigeration.
Coupled Model (or coupled atmosphere-ocean model) - In the context of climate modeling this
usually refers to a numerical model which simulates both atmospheric and oceanic motions and
temperatures and which takes into account the effects of each component on the other.
Crop Moisture Index - In 1968, Palmer developed the index to assess short-term crop water
conditions and needs across major crop-producing regions. This index is a useful tool in
forecasting short-term drought conditions.
(See Palmer Drought Severity & Crop Moisture Indices)
Cyclone - In general use the term cyclone is applied to any storm, especially violent, small scale
circulations such as tornados, waterspouts, and dust devils. In meteorology, the term refers to a
type of atmospheric disturbance centered around a low-pressure center that often results in
stormy weather. In common practice the term cyclone, and low, are used interchangeably and are
frequently referred to as storms. In the Northern Hemisphere the air rapidly circulates
counterclockwise and in the Southern Hemisphere clockwise. Tropical cyclones with sustained
winds above 73 miles per hour are known as hurricanes in the North Atlantic Ocean, Caribbean
Sea, Gulf of Mexico and the Eastern North Pacific (east of the date line) and cyclones in the
Indian Ocean. They are known as typhoons in other areas or the world. Both mid-latitude and
tropical storms serve an important function in transferring warmth away from the tropics to the
Decadal - A consecutive ten year period.
Degree Day - For any individual day, degree days indicate how far that day's average
temperature departed from 65oF. Heating Degree Days measure heating energy
demand. It is a measure to indicate how far the average temperature fell below 65oF. Similarly,
Cooling Degree Days's, which measure cooling energy demand, indicate how far the temperature
averaged above 65oF. In both cases, smaller values represent less fuel demand, but values below
0 are set equal to 0, because energy demand cannot be negative. Furthermore, since energy
demand is cumulative, degree day totals for periods exceeding 1 day are simply the sum of each
individual day's degree day total. For example, if some location had a mean temperature of 60oF
on day 1 and 80oF on day 2, there would be 5 HDD's for day 1 (65 minus 60) and 0 for day 2 (65
minus 80, set to 0). For the day 1 + day 2 period, the HDD total would be 5 + 0 = 5. In contrast,
there would be 0 CDD's for day 1 (60 minus 65, reset to 0), 15 CDD's for day 2 (80 minus 65),
resulting in a 2-day CDD total of 0 + 15 = 15.
Dew Point - The point at which the air at a certain temperature contains all the moisture possible
without precipitation occurring. When the dew point is 65oF, one begins to feel the humidity.
The higher the temperature associated with the dew point, the more uncomfortable one feels.
Dobson Unit - Unit used to measure the abundance of ozone in the atmosphere. One Dobson
unit is the equivalent of 2.69 x 1016 molecules of ozone/cm2.
Doppler radar - Radar that measures speed and direction of a moving object, such as water or ice
particles, birds, and insects.
Drought - Drought is a deficiency of moisture that results in adverse impacts on people,
animals, or vegetation over a sizeable area. NOAA together with its partners provides short- and
long-term Drought Assessments.
Drought Assessments - At the end of each month, CPC issues a long-term seasonal drought
assessment. On Thursdays of each week, the CPC together with NOAA National Climatic Data
Center, the United States Department of Agriculture, and the National Drought Mitigation Center
in Lincoln, Nebraska, issues a weekly drought assessment called the United States Drought
Monitor. These assessments review national drought conditions and indicate potential impacts
for various economic sectors, such as agriculture and forestry.
El Niño - El Niño, a phase of ENSO, is a periodic warming of surface ocean waters in the
eastern tropical Pacific along with a shift in convection in the western Pacific further east than
the climatological average. These conditions affect weather patterns around the world. El Niño
episodes occur roughly every four-to-five years and can last up to 12-to-18 months. The preliminary CPC definition of El Niño
is a phenomenon in the equatorial Pacific Ocean characterized by a positive sea surface temperature departure from normal (for the 1971-2000 base period), averaged over three months, greater than
or equal in magnitude to 0.5oC in a region defined by 120oW-170oW and 5oN-5oS (commonly referred to as Niņo 3.4).
El Niño, which would appear off the coast of Peru around Christmas time,
is Spanish for "the boy" referring to the Christ child.
Ensemble Forecast - Multiple predictions from an ensemble of slightly different initial
conditions and/or various versions of models. The objectives is to improve the accuracy of the
forecast through averaging the various forecasts, which eliminates non-predictable components,
and to provide reliable information on forecast uncertainties from the diversity amongst
ensemble members. Forecasters use this tool to measure the likelihood of a forecast.
ENSO (El Niño-Southern Oscillation) - Originally, ENSO referred to El Niño/ Southern
Oscillation, or the combined atmosphere/ocean system during an El Niño warm event. The
ENSO cycle includes La Niña and El Niño phases as well as neutral phases, or ENSO cycle, of
the coupled atmosphere/ocean system though sometimes it is still used as originally defined. The
Southern Oscillation is quantified by the Southern Oscillation Index (SOI).
ENSO Diagnostic Discussion (formerly ENSO Advisory) - The CPC issues the ENSO
Diagnostic Discussion around the middle of the month. The discussion addresses the current
oceanic and atmospheric conditions in the Pacific and the seasonal climate outlook for the
following one to three seasons.
Evaporation - The physical process by which a liquid or solid is changed to a gas; the opposite
Excessive Heat Outlook - This CPC product, a combination of temperature and humidity over
a certain number of days, is designed to provide an indication of areas of the country where
people and animals may need to take precautions against the heat during May to November.
Experimental Products - An experimental product is in the final stages of testing and
evaluation. If the product proves accurate and valuable to users then the next step is to make it an
Extratropical - In meteorology, the area north of the Tropic of Cancer and the area south of the
Tropic of Capricorn. In other words, the area outside the tropics.
Forecasts (Synonymous with predictions and outlooks) - A weather forecast, or prediction, is an
estimation based on special knowledge of the future state of the atmosphere with respect to
temperature, precipitation, and wind. Weather forecasts are now routinely provided for up to 14
days in advance and outlooks for seasonal and longer timescales.
GCMs (General Circulation Models) - These computer simulations reproduce the Earth's
weather patterns and can be used to predict change in the weather and climate.
Global Warming - Certain natural and human-produced gases prevent the sun's energy from
escaping back to space leading to an overall rise in the temperature of the Earth's atmosphere.
GMT - Greenwich Mean Time
Greenhouse Effect - The atmosphere allows solar radiation to reach the earth relatively easily.
The atmosphere absorbs the infrared radiation emitted by the Earth's surface and radiates it back
to the Earth in much the same way a greenhouse traps heat as the sun's rays pass through the
glass, and the heat generated does not pass back through the glass. The "greenhouse effect"
causes the surface of the Earth to be much warmer that it would be without the atmosphere
60oF). Without the greenhouse effect, life as we know it might not exist on Earth.
Greenhouse Gas - Certain gases, such as water vapor, carbon dioxide, and methane, that more
effectively trap heat affecting the Earth's surface temperature.
Greenwich Mean Time - GMT is the global standard for time that was established in 1884
when delegates from 27 nations met in Washington, DC for the Meridian Conference and agreed on a system basically the same as that now in use. Civilian designations are typically three letter
abbreviations (e.g. EST) for most time zones. Military designations use each letter of the
alphabet (except 'J') and are known by their phonetic equivalent. For example, Greenwich Mean
Time (civilian) or Z = Zulu (military and aviation). Universal Time Coordinated (UTC) has been substituted for GMT.
Growing Degree Days - A heat index that relates the development of plants, insects, and disease
organisms to environmental air temperature. The index varies depending on whether it is a cool,
warm, or very warm season plant. For example, a corn growing degree day (GDD) is an index
used to express crop maturity. The index is computed by subtracting a base temperature of 50oF from the average of the maximum and minimum temperatures for the day. Minimum
temperatures less than 50oF are set to 50, and maximum temperatures greater than 86oF are set to
86. These substitutions indicate that no appreciable growth is detected with temperatures lower
than 50 or greater than 86. If the maximum and minimum temperatures were 85 and 52, you
would calculate the GDD by ((85+52/2) - 50) = 18.5 GDD.
Hazards Assessment - CPC's Hazards Assessment provides emergency managers, planners,
forecasters and the public advance notice of potential hazards related to climate, weather and hydrological events.
Heating Degree Days - A form of degree-day used to estimate energy requirements for heating.
HectoPascals (hPa) - 1 hPA= 1 millibar, a unit of pressure.
500 hPa - At this height above sea level half the mass of the atmosphere lies above and below as
measured in pressure units. This area is important for understanding surface weather for upper
air storms tend to be steered in the direction of the winds at this level and are highly correlated
with surface weather.
Hurricane - See Cyclone
Hydrology - The scientific study of precipitation, evaporation, distribution, and effects of water
on the Earth's surface, in the soil and rocks, and in the atmosphere.
Intraseasonal Oscillations - Variability on a timescale less than a season. One example is the
Jet Stream - Strong winds concentrated within a narrow zone in the atmosphere in the upper
troposphere, about 30,000 feet aloft that generally move in an easterly direction that drive
weather systems around the globe. In North America jet streams are more pronounced in winter.
Kelvin Waves - Fluctuations in wind speed at the ocean surface at the Equator result in eastward
propagating waves, known as Kelvin Waves. Kelvin Waves cause variations in the depth of the
oceanic thermocline, the boundary between warm waters in the upper ocean and cold waters in
the deep ocean. They play an important role in monitoring and predicting El Niño episodes.
La Niña - La Niña, a phase of ENSO, is a periodic cooling of surface ocean waters in the eastern
tropical Pacific along with a shift in convection in the western Pacific further west than the
climatological average. These conditions affect weather patterns around the world. The preliminary CPC definition of La Niņa is a phenomenon in
the equatorial Pacific Ocean characterized by a negative sea surface temperature departure from normal (for the 1971-2000 base period), averaged over three
months, greater than or equal in magnitude to 0.5oC in a region defined by 150oW-160oE and 5oN-5oS (commonly referred to as Niņo 4).
Long Wave (or Planetary Wave) - In meteorology, a long wave in atmospheric circulation in
the major belt of westerlies has different characteristics than rapidly moving storms nearer the
Low Level Jet - A jet stream that is typically found in the lower 2-3 km, or approximately 6,000
to 9, 500 feet, of the troposphere.
Madden-Julian Oscillation (MJO) - Tropical rainfall exhibits strong variability on time scales
shorter than the seasonal El Niño-Southern Oscillation (ENSO). These fluctuations in tropical
rainfall often go through an entire cycle in 30-60 days, and are referred to as the Madden-Julian
Oscillation or intraseasonal oscillations. The intraseasonal oscillations are a naturally occurring
component of our coupled ocean-atmosphere system. They significantly affect the atmospheric
circulation throughout the global Tropics and subtropics, and also strongly affect the wintertime
jet stream and atmospheric circulation features over the North Pacific and western North
America. As a result, they have an important impact on storminess and temperatures over the
United States. During the summer these oscillations have a modulating effect on hurricane
activity in both the Pacific and Atlantic basins.
Mean - The arithmetic average, or the middle point between two extremes.
Meteorology - The scientific study of the physics, chemistry, and dynamics of the Earth's
atmosphere, especially weather and climate.
Millibar - A pressure unit of 1000 dynes cm-2, often used for reporting atmospheric pressure
where one millibar equals 1 hectopascal (hPA).
Monsoons - Seasonal winds. They are caused primarily by the greater annual variation in air
temperature over large land surfaces compared to ocean surfaces though other factors like
land-relief are important.
MRF - Medium Range Forecast, usually a reference to a numeric model that extends one to two
weeks into the future.
MRF-Based 8-day Guidance - Based on the MRF model output statistics the Guidance gives a
temperature forecast (max and min) with actual values (e.g. high of 50 low of 40) for the next 7
days plus probability of precipitation, and mean wind speed and cloudiness in easy-to-understand
and use text format.
NAO - North Atlantic Oscillation
NAO Index - This index measures the anomalies in sea level pressure between the Icelandic
low pressure system and the Azores high pressure system. When the difference is positive the
northeastern United States sees an increase in temperature and a decrease in snow days; the
central US has increased precipitation, the North Sea has an increase in storms; and Norway
along with Northern Europe has warmer temperatures and increased precipitation. When the
difference is negative the Tropical Atlantic and Gulf coast have increased number of strong
hurricanes; northern Europe is drier, and Turkey along with other Mediterranean countries has
National Centers for Environmental Prediction (NCEP) - The National Centers for
Environmental Prediction (NCEP), an arm of the NOAA's National Weather Service (NWS)
provides national and international weather and climate guidance products to National Weather
Service field offices, government agencies, emergency managers, private sector meteorologists,
and meteorological organizations and societies throughout the world. The nine centers are:
Aviation Weather Center, Climate Prediction Center, Environmental Modeling Center,
Hydrometeorological Prediction Center, Marine Prediction Center, NCEP Central Operations,
Space Environment Center, Storm Prediction Center and Tropical Prediction Center (formerly
the National Hurricane Center). (See http://www.ncep.noaa.gov)
National Climatic Data Center (NCDC) - NCDC maintains the world's largest active archive
of weather data. NCDC produces numerous climate publications and responds to data requests
from all over the world. (See http://www.ncdc.noaa.gov)
National Oceanic and Atmospheric Administration (NOAA) - NOAA's historical role has
been to predict environmental changes, protect life and property, provide decision makers with
reliable scientific information, and foster global environmental stewardship. Today NOAA's
mission remains unchanged as it describes and predicts changes in the Earth's environment, and
conserves and wisely manages the Nation's coastal and marine resources. (See http://www.noaa.gov)
National Weather Service (NWS) - The National Weather Service (NWS) - provides weather,
hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent
waters and ocean areas, for the protection of life and property and the enhancement of the
national economy. NWS data and products form a national information database and
infrastructure which can be used by other governmental agencies, the private sector, the public,
and the global community. (See http://www.nws.noaa.gov)
NCEP - National Centers for Environmental Prediction
Niño 1+ 2, 3, 3.4, and 4 - In monitoring the equatorial tropical Pacific for the phases of the
ENSO cycle, the area has been divided into 4 sections:
- Niño 1+2 (0o-10o South) (90o West-80o West)
- Niño 3 (5o North-5o South) (150o West-90o West)
- Niño 4 (5o North-5o South) (160o East-150o West)
- Niño 3.4 (5o North-5o South) (170o-120o West)
The reason for this is that major atmospheric circulation impacts are related to changes in the
pattern of convection in these regions. The Niñno 3.4 and Niñno 4 regions encompass the
area where slight increases or decreases in SSTs can have a big impact on where convection is
found in the western and central Pacific and are the key areas for monitoring and predicting
NOAA - National Oceanic and Atmospheric Administration
Normal - To understand whether precipitation and temperature is above or below normal for
seasons and longer timescales, normal is defined as the average weather over 30 years. These
averages are recalculated every ten years. The National Weather Service has just recalculated the
baseline period for normal from 1961 to 1990 to 1971 to 2000. Since the cool decade of the
1960's has been replaced with the mild 1990's, normal temperatures in many areas have
North Atlantic Oscillation- The NAO is a large-scale fluctuation in atmospheric pressure
between the subtropical high pressure system located near the Azores in the Atlantic Ocean and
the sub-polar low pressure system near Iceland and is quantified in the NAO Index. The surface
pressure drives surface winds and wintertime storms from west to east across the North Atlantic
affecting climate from New England to western Europe as far eastward as central Siberia and
eastern Mediterranean and southward to West Africa
Numerical Forecasting (Also called mathematical forecasting, dynamical forecasting, physical
forecasting, and numerical weather prediction) - A computer forecast or prediction based on
equations governing the motions and the forces affecting motion of fluids. The equations are
based, or initialized, on specified weather or climate conditions at a certain place and time.
NWS - National Weather Service
Office of Global Programs (OGP) - The Office of Global Programs (OGP) sponsors focused
scientific research, within approximately eleven research elements, aimed at understanding
climate variability and its predictability. Through studies in these areas, researchers coordinate
activities that jointly contribute to improved predictions and assessments of climate variability
over a continuum of timescales from season to season, year to year, and over the course of a
decade and beyond.
OLR - Outgoing Longwave Radiation is a polar satellite derived measurement of the radiative
character of energy radiated from the warmer earth surface to cooler space. This measurement
provides information on cloud-top temperature which can be used to estimate tropical
precipitation amounts which is important in forecasting weather and climate.
Operational Products - Products and data that have been fully tested and evaluated that are
produced on a regular and ongoing basis.
Oscillations - A shift in position of various high and low pressure systems that in climate terms
is usually defined as an index (i.e., a single numerically-derived number, that represents the
distribution of temperature and pressure over a wide ocean area, such as the El Niño-Southern
Oscillation, North Atlantic Oscillation, and Pacific Decadal Oscillation).
Ozone - A molecule consisting of three oxygen atoms that is formed by a reaction of oxygen
and ultraviolet radiation. In the stratosphere, ozone has beneficial properties where it forms an
ozone shield that prevents dangerous radiation from reaching the Earth's surface. Closer to the
planet's surface, ozone is considered an air pollutant that adversely affects humans, plants and
animals as well as a greenhouse gas.
Ozone Hole - A severe depletion of stratospheric ozone over Antarctica that occurs each spring.
The possibility exists that a hole could form over the Arctic as well. The depletion is caused by a
chemical reaction involving ozone and chlorine, primarily from human produced sources, cloud
particles, and low temperatures.
Pacific Decadal Oscillation - A recently described pattern of climate variation similar to ENSO
though on a timescale of decades and not seasons. It is characterized by SST anomalies of one
sign in the north-central Pacific and SST anomalies of another sign to the north and east near the
Aleutians and the Gulf of Alaska. It primarily affects weather patterns and sea surface
temperatures in the Pacific Northwest, Alaska, and northern Pacific Islands. Two main
characteristics distinguish PDO from El Niño/Southern Oscillation (ENSO): first, 20th century
PDO "events" persisted for 20-to-30 years, while typical ENSO events persisted for 6 to 18
months; second, the climatic fingerprints of the PDO are most visible in the North Pacific/North
American sector, while secondary signatures exist in the tropics- the opposite is true for ENSO.
Several independent studies found evidence of just two full PDO cycles in the past century:
cool" PDO regimes prevailed from 1890-1924 and again from 1947-1976, while "warm" PDO
regimes dominated from 1925-1946 and from 1977 through (at least) the mid-1990's. Causes for
the PDO are not currently known. Likewise, the potential predictability for this climate
oscillation are not known.
Palmer Drought Severity Index (PDSI) - An index that compares the actual amount of
precipitation received in an area during a specified period with the normal or average amount
expected during that same period. It was developed to measure lack of moisture over a relatively
long period of time and is based on the supply and demand concept of a water balance equation.
Included in the equation are amount of evaporation, soil recharge, and runoff and
temperature and precipitation data.
PRCP - Abbreviation for precipitation.
Probability - A chance, or likelihood, that a certain event might happen.
Relative humidity - An estimate of the amount of moisture in the air relative to the amount of
moisture that the air can hold at a specific temperature. For example, if it's 70oF near dawn on a
foggy summer morning, the relative humidity is near 100%. During the afternoon the
temperature soars to 95oF and the fog disappears. The moisture in the atmosphere has not
changed appreciably, but the relative humidity drops to 44% because the air has the capacity to
hold much more moisture at a temperature of 95oF than it does at 70oF. But even when the
relative humidity is "low" at 44%, it's a very humid day when the temperature is 95oF. For this
reason, a better measure of comfort is dew point.
Retrogression or retrograde motion means motion that is backwards from the usual way things move in the Northern Hemisphere extratropics - which is from west to east.
In meterology, the term is used in relation to atmospheric waves or pressure systems. When meterologists say that a pattern will retrograde, they mean that the troughs
and ridges will end up further west than they were previously. Normal motion (over the United States) is progressive, or prograde, which means (weather systems move)
from west to east.
Sea Surface Temperatures (SSTs) - The term refers to the mean temperature of the ocean in
the upper few meters.
SOI (Southern Oscillation Index) - SOI is based on the (atmospheric) pressure difference
between Tahiti and Darwin, Australia. It is highly correlated with tropical sea surface
temperature anomaly indices recorded in Niño3.
SSTs - Sea Surface Temperatures
Stratosphere - The region of the atmosphere extending from the top of the troposphere to the
base of the mesosphere, an important area for monitoring stratospheric ozone.
Stratospheric ozone - In the stratosphere, ozone has beneficial properties where it forms an
ozone shield that prevents dangerous radiation from reaching the Earth's surface. Recently, it
was discovered that in certain parts of the world, especially over the poles, stratospheric ozone
was disappearing creating an ozone hole.
Subtropical - A climate zone adjacent to the tropics with warm temperatures and little rainfall.
Synoptic scale - Used to classify large-scale weather systems more than 200 miles across.
Teleconnection - A strong statistical relationship between weather in different parts of the
globe. For example, there appears to be a teleconnection between the tropics and North America
during El Niño.
TEMP - Abbreviation for temperature, given in Fahrenheit for CPC products.
Thermocline - As one descends from the surface of the ocean, the temperature remains nearly
the same as it was at the surface, but at a certain depth temperature starts decreasing rapidly with
depth. This boundary is called the thermocline. In studying the tropical Pacific Ocean, the depth
of 20oC water ("the 20oC isotherm") is often used as a proxy for the depth of the thermocline.
Along the equator, the 20oC isotherm is typically located at about 50m depth in the eastern
Pacific, sloping downwards to about 150 m in the western Pacific.
Troposphere - The lowest portion of the atmosphere which lies next to the earth's surface
where most weather occurs.
Typhoon - See Cyclone
Ultraviolet (UV) (or Ultraviolet Radiation) - Ultraviolet radiation from the sun plays a role in
the formation of the ozone layer by acting as a catalyst for a chemical reaction that breaks apart
oxygen molecules which then recombine to form ozone. The absorption of UV by stratospheric
ozone and atmospheric oxygen prevents very little ultraviolet radiation to reach earth's surfaces
where it can detrimental effects on human health and property.
Upper-level (or upper air) - In weather observing, the term applies to the portion of the
atmosphere that is above the lower troposphere, generally 850 hPa and above.
Upwelling - In ocean dynamics, the upward motion of sub-surface water toward the surface of
the ocean. This is often a source of cold, nutrient-rich water. Strong upwelling occurs along the
equator where easterly winds are present. Upwelling also can occur along coastlines, and is
important to fisheries and birds in California and Peru.
UTC - Universal Time Coordinated is the same as Greenwich Mean Time
Wind Chill - The portion of the cooling of the human body caused by air motion. Wind chill
becomes important for human health as air motion accelerates the rate of heat loss from a human
body, especially when temperatures are below 45oF.
Wind Chill Index - A means of quantifying the threat of heat loss from the human body during
windy and cold conditions.
Z=Zulu - See GMT or Greenwich Mean Time
* Sources for definitions include: The American Heritage Dictionary, The New York Times Weather Report,
American Meteorological Society's Glossary of Meteorology (2000), and the Internet.